Generally speaking, a conventional power drill comprises a spindle and a rear bearing mounted on the rear portion of the spindle. A fixed gear wheel is connected to a gearbox and a movable gear wheel is fixed to the spindle. An impact adjustment member provided on the front portion of the spindle comprises an impact adjustment cover, an impact fork connected to the impact adjustment cover and a sleeve connected to the impact fork. The front interior of the sleeve slidably contacts the exterior periphery of the spindle and is mounted on the front portion of the spindle. The impact sleeve is provided with a groove and a stroke adjustment member is disposed between the movable gear wheel and the sleeve. On the stroke adjustment member is a protruding block. A stopper is disposed on the front portion of the spindle. As disclosed in U.S. Pat. No. 6,202,759, U.S. Pat. No. B1-6,196,076 and U.S. Pat. No.5,451,127, dispersed steel balls with retainers are used to reduce the friction between the impact adjustment member and the shoulder of the output axle. The resulting construction is complex with many parts and demands a large distance between the front end of the sleeve and the front end of the stopper. The larger radial runout of the output axle due to the bigger ratio L2/(L1+L2) (where L1 is the distance between the back end of the rear bearing and the front end of the sleeve and L2 is the distance between the front end of the sleeve and the front end of the stopper) results in an inaccurate diameter and reaming and difficulty in hole positioning.